Game guide:
WASD to move, left click to shoot and right click for a super-shoot. In the top left corner you have violence medallions. You can find more in the maps by exploring. You fill them by damaging and killing your enemies. You need a full medallion to be able to use your super-shoot, which will empty it. If you die while one of them is full, you will use one to revive from the remains of the last enemy killed. At the bottom left you have the dash-meter; once full, press shift to dash. Works on air too! Top right you can check your ammo: you start with 2 bullets plus an additional 2 per kill. Note that the super-shoot do not consume your ammo. If you run out of ammo you can press Q to punch an enemy to death. At the bottom you'll find the remaining time. You have 10 seconds to live, and each time you kill the time resets, so don't stop killing and keep an eye on the amount of enemies so you don't run out of fodder to keep living. You will find energy shields along your journey: break they purple-glowing antennas that power them to open your path. Lastly, you will find some enemies with an energy shield. In order to kill them you'll have to use your super-shoot or break their shields with a punch. But keep an eye on these enemies, because some shield antennas are also protected by energy shields, and one of these enemies from the surrounding area has the key: kill them to access the antenna so you can destroy it.
As for the levels:
Level 1. You start on a pool of your own blood and open your way through a destroyed city to the enemy base located at the other side of a cave complex accessed through the mall. You'll find your first energy shield here. One generator is at the end of the road below, and the other is in an alley in front of the energy shield next to your first extra medallion. Once broken, advance to the parking lot and enter the mall to finish the level.
Level 2. Here you'll find your first shielded enemy. The starting room is blocked by an energy shield with a single antenna protected by its own shield. One of the three shielded soldiers hold its key, so kill them all (or until you kill the right one) and the antenna will unlock for you to destroy it. Once you've exited this first room you'll find yourself in a bigger room with 2 stories, but you can't access the second floor from here. You'll see right in front of you a door with 2 shielded antennas, and 2 ways on left and right side, but the right way is locked by another energy shield that you can't open from here. Go left and kill every shielded enemy you see. Here, you'll see a (non-functioning) mechanical ramp leading the second floor. Go up and keep killing every shielded soldier (you don't know which one holds the key for the antennas). From here you'll see a rock barricade and a ramp that will allow to jump over it and reach the main hall from the second floor this time. On the right side you'll find a ramp that allows you to jump over the void and reach the second floor of the right area. Keep going and killing every shielded soldier in the broken mechanical ramp room and advance rapidly to the main hall from the main floor. You'll see the antenna locking the energy shield from before. Now you'll be able to open the way by destroying the two antennas that were previously shielded. Advance through the room full of enemies into a hole on the ground that will lead you to the cave system and end the level.
Level 3. As you turn the corner and see the true extension of the cave system you'll see 2 ways. If you go straight (or slightly to the right, depends on your point of view) you'll see your goal: another energy shield blocking your way to the enemy base. There is also another energy shield that holds a medallion (with it you'll have 3 medallions if you picked the one in the first level, the max amount in the game). You'll have to go left and jump many platforms using your dash to be able to open the way. After the first jump you'll see two paths. One of them is shorter and easier which is the one in the front. If you turn to the left and jump on the longer path, you'll see an antenna behind a rock pillar. Break it to open the medallion energy shield. This path will lead you to the top part of the shorter path after this detour. Either way, keep advancing until you reach the long platform on the cave walls. This platform will lead you to the top of the cave. Here, there is the shielded antenna that blocks your exit. The soldier that has the key to that antenna is in this specific section on the top of the cave. Kill them, break the antenna and jump down to where you started. Be careful: you'll need to be quick to kill something when you land or you'll run out of time. On the cave wall you'll find the last medallion if you opened the door. Take it and advance through the cave to the exit of the level.
Level 4. This level is fairly short but it holds a challenge. Advance to the arena with a robot on the center and fight it. On the sides of the arena there are teleport stations that will spawn enemies nonstop, which will help you stay alive during the fight against the Sentinel, the final boss of this game. It can attack you melee with its chainsaw, with its plasma rifle dealing lots of damage and with a missile shower. If you want a real challenge on this fight, play it on maximum difficulty without any of the additional medallions. After killing it, a door that was previously closed will open up, revealing the central control unit of the enemy base. Destroy it to stop your enemies from detonating the bombs that will kill your son.